Magnetic proximity switches, also known as limit switches, are commonly used for linear position sensing. Generally, magnetic proximity systems typically include a target and a sensor. In one example, the target passes within a predetermined range of the sensor, the magnetic flux generated by the target, such as target magnet, causes the switch to close.
FIGS. 1A and 1B depict a conventional proximity switch 10 disposed within a switch box 12 operatively coupled to a rotary actuator 14 having a shaft 16. The switch box 12 includes an opening through which the shaft 16 passes. The switch box 12 houses both the conventional proximity switch 10 and a target carrier 18, such as a disk, having a target magnet 20 disposed thereon. The target carrier 18 also includes an opening for receiving the shaft 16, such that when the shaft 16 is rotated the target carrier 18 is rotated. To set the proximity switch 10 to trigger at a certain point of rotation of the actuator 14, the actuator 14, and, thus, the shaft 16, is rotated to that desired point. The proximity switch 10 is stationary; it never moves even when the shaft 16 moves.
When the shaft 16 is rotated, the target carrier 18 rotates to move the target magnet 20 into a sensing area of the proximity switch 10. When the target magnet 20 moves into the sensing area of the proximity switch 10, the proximity switch 10 is attracted to the target magnet 20, causing the proximity switch 10 to change states.
More specifically, and referring now to FIGS. 2-5, the conventional target magnet 20 is depicted in various states. For example, FIG. 2 depicts the conventional target magnet 20 outside of the sensing area SA (FIG. 5) of the proximity switch 10, and the proximity switch 10 is in an inactivated state. Upon movement of the target magnet 20 (such as via rotation of the shaft 16 by the actuator 14, which in turn rotates the target carrier 18) into the sensing area SA of the proximity switch 10, the proximity switch 10 is attracted to the target magnet 20 having an end with a magnetic polarity opposite the magnetic polarity of an end of the proximity switch 10. So configured, this arrangement causes the proximity switch 10 to move to an activated state, as depicted in FIG. 3.
However, once the proximity switch 10 is triggered or activated by the target magnet 20 to the activated state, stopping and immediately reversing the direction of the target magnet 20 does not immediately reset the proximity switch 10. Instead, due to hysteresis effects, the proximity switch 10 remains in the activated state until the target magnet 20 moves an amount sufficient to break the magnetic field outside of the sensing area SA. When this occurs, the proximity switch 10 is released back to the unactivated state, as depicted in FIG. 4.
Referring now to FIG. 5, the target magnet 20 is polarized with a pole N facing the opposite pole S of the proximity switch's sensing magnet 11 disposed adjacent to the target magnet 20. When the target magnet 20 is moved into the sensing area SA of the proximity switch 10, the proximity switch 10 is attracted to the target magnet 20, is actuated to the activated state, and remains in the activated state until the target magnet 20 moves out of the sensing area SA of the proximity switch 20, plus hysteresis. This hysteresis can significantly delay the return of the proximity switch 10 back to an unactivated state, for example. This also reduces the accuracy of determining a rotational position of the actuator 14 at any one time.